# Lithoxyl Overview¶

Lithoxyl is next-generation logging and instrumentation for Python. This practical tutorial walks new users through the fundamentals necessary to get up and running with Lithoxyl in under 10 minutes.

## Motivating factors¶

Lithoxyl began as a response to the tired traditions of logging. Traditions that included omission, procrastination, and only adding it once things break.

Logging is not the last step anymore. Lithoxyl makes instrumentation worthwhile from day 1, so all your projects are designed for introspection. Lithoxyl achieves this by taking full advantage of Python’s rich syntax and runtime, providing features ranging from metrics collection to structured logging to interactive debugging hooks.

The Lithoxyl approach is practical. After running pip install lithoxyl, integrating Lithoxyl comes down to two steps: instrumentation and configuration. First, instrumentation.

## Instrumenting with Actions¶

With Lithoxyl, all instrumentation, including logging, starts with knowing your application. We want to find the important parts of your application and wrap them in microtransactions, called Actions.

Much more than print statements, Actions are lightweight objects that track the state of code execution, from timing information to uncaught exceptions. Each Action also has a name and a level, to enable aggregation and filtering.

Actions are created with Loggers. We get into creating and configuring Loggers later in the overview, but here’s a basic example of creating an info-level Action with a preconfigured Logger:

import backend           # some convenient backend logic for brevity
from log import app_log  # preconfigured Lithoxyl Logger

def create_entry(name):
name = name.strip()
return True


As you can see, the transactionality of Actions translates well to Python’s with context manager syntax. A single line of logging code succinctly records the beginning and ending of this code block. Even better, there’s no chance of missing an unexpected exception. For instance, if name is not a string, and .strip() raises an AttributeError, then that exception is guaranteed to be captured and recorded.

You can do so much more with actions. Using dictionary syntax, arbitrary data can be added to the action. And while actions finish with a success status and autogenerate a message if no exception is raised, failures and exceptions can also be set manually:

import backend
from log import app_log

def set_entry_state(name, state):

with app_log.info('setting entry state') as act:
act['name'] = name
status = backend.STATE_MAP[state.lower()]
success = backend.set_entry_state(name, state)
if not success:
act.failure('set {name} status to {state} failed', state=state)

return success


As seen above, actions can also have a custom completion message, which supports templating with new-style formatting syntax, using data from within the action’s data map (name), as well as arguments and keyword arguments (state).

Note

Even if message formatting fails, the log message will still be recorded. Only the failing segments will be left unformatted. As a rule, Lithoxyl degrades gracefully, to minimize impact to your application’s primary functionality.

Furthermore, in cases like this, where you want the whole function logged, you can use the logger’s wrap() method.:

import backend
from log import app_log

@app_log.wrap('critical', inject_as='act')
def delete_entry(name, act):
try:
ret = backend.delete_entry_by_name(name.strip())
except backend.EntryNotFound:
# log soft error, let other exceptions raise through
act.failure('no entry with name: {}', name)
ret = False
return ret


Note the decorator syntax, as well as the ability to inject the action as one of the arguments of the function. This reduces the instrumentation’s code footprint even further.

That about covers creating and interacting with actions. Now we turn to the origin and destination of the actions we create and populate: Loggers and Sinks.

## Creating Loggers¶

Actions make up most of an application’s interaction with Lithoxyl, but it would not be very easy to create an Action without a Logger.

As we learned above, before an Action can be populated, it must be created, and Actions are created through Logger. As for the Logger itself, here is how it is created:

from lithoxyl import Logger

app_log = Logger('entry_system')


Like that, the Logger we’ve been using above is ready to be imported. A Logger is a lightweight, simple object, requiring only a name. They are designed to be created once, configured, and imported by other modules. That said, they are conceptually very useful.

Loggers generally correspond to parts or aspects of the application. Small- to medium-sized applications can be fully instrumented with just one Logger, but as applications grow, they tend to add aspects. For example, if file access grows increasingly important to an application, it would make sense to add a dedicated low-level log just for instrumenting file access:

file_log = Logger('file_access')


In short, Loggers themselves are simple, and designed to be fit to your application, no matter how many aspects it may have. On their own, they are conceptually useful, but without Sinks, they are all potential.

## Configuring Sinks¶

So far, we have discovered two uses of the Lithoxyl Logger:

• Creating actions
• Segmenting and naming aspects of an application

Now, we are ready to add the third: publishing log events to the appropriate handlers, called Sinks. Actions can carry all manner of messages and measurements. That variety is only surpassed by the Sinks, which handle aggregation and persistence, through log files, network streams, and much more. Before getting into those complexities, let’s configure our app_log with a simple but very useful sink:

from lithoxyl import AggregateSink

agg_sink = AggregateSink(limit=100)


Now, by adding an instance of the AggregateSink to the app_log, we have a technically complete system. At any given point after this, the last 100 events that passed through our application log will be available inside agg_sink. However, AggregateSinks only provide in-memory storage, meaning data must be pulled out, either through a monitoring thread or network service. Most developers expect persistent logging to streams (stdout/stderr) and files. Lithoxyl is more than capable.

## Logging Sensibly¶

For developers who want a sensible and practical default Sink, Lithoxyl provides the SensibleSink. The Sensible Suite chapter has a full introduction, so let’s just cover the basics.

The Sensible approach has 3 steps:

1. Filter - Optionally ignore events for a given Sink.
2. Format - Convert an event into a string.
3. Emit - Output the formatted string to a file, database, network, etc.

While totally pluggable and overridable, the Sensible suite ships with types for each of these:

from lithoxyl import (SensibleFilter,
SensibleFormatter,
StreamEmitter,
SensibleSink)

# Create a filter that controls output verbosity
fltr = SensibleFilter(success='critical',
failure='info',
exception='debug')

# Create a simple formatter with just two bits of info:
# The time since startup/import and end event message.
# These are just two of the built-in "fields",
# and the syntax is new-style string formatting syntax.
fmtr = SensibleFormatter('+{import_delta_s} - {end_message}')

# Create an emitter to write to stderr. 'stdout' and open file objects
# also behave predictably.
emtr = StreamEmitter('stderr')

# Tie them all together. Note that filters accepts an iterable
sink = SensibleSink(filters=[fltr], formatter=fmtr, emitter=emtr)

# Add the sink to app_log, a vanilla Logger created above


In these six lines of code, using only built-in Lithoxyl types, we create a filter, formatter, and emitter, then we bind them all together with a SensibleSink. The output is first filtered by our SensibleFilter, which only shows critical-level successes and info-level failures, but shows all exceptions. Our SensibleFormatter provides a simple but practical output, giving us a play-by-play timing and message. That message is output to stderr by our StreamEmitter. Just don’t forget to add our newly-created SensibleSink to the app_log.

As configured, the app_log will now write to stderr output that looks like:

+0.015255 - "load credential succeeded"
+0.179199 - "client authorization succeeded"